(1) Field of the Invention
The invention relates to a screen, and especially relates to a projection screen.
(2) Description of the Prior Art
FIG. 1 is a conventional reflective projection screen 100, including a substrate 110, a light transparent layer 120, a light reflection layer 130, a light absorbing layer 140, an light anti-reflection layer 150, and a back protection layer 160. The reflective projection screen 100 is used to reflect image light beams L1 and L2 from a projector 200.
The light anti-reflection layer 150 is attached to the front surface of the substrate 110 and faces the projector 200 to ensure that the image light beams L1,L2 are not reflected by the surface of the substrate 110. The light transparent layer 120 is disposed at the back of the substrate 110, and the section of the light transparent layer 120 is similar to a plurality of ladder units connected to each other. Each of the ladder units has a narrow upside and a wide underside, and the underside is connected with the substrate 110. The light reflection layer 130 is formed above the upside of each of the ladder units of the light transparent layer 120. The light absorbing layer 140 is composed by black particles 140a or light absorbing material filled between adjacent ladder units of the light transparent layer 120.
For the average index of refraction of the light absorbing layer 140 is smaller than the index of refraction of the light transparent layer 120, the image light beams L1,L2 have opportunities to be totally reflected at the boundary interface between the light transparent layer 120 and the light absorbing layer 140. As FIG. 1 shows, after entering the projection screen 100 from the projector 200, the image light beams L1,L2 are refracted by the surface of the substrate 110, and then enter into a space encircled by the light reflection layer 130 and the two interfaces between the light transparent layer 120 and the two adjacent light absorbing layers 140. The image light beams L1, L2 exit out of the projection screen 100 by total reflection of the two interfaces and mirror reflection of the light reflection layer 130.
When the environment light G enters obliquely into the projection screen 100, for the incident angle of the environment light G entering the boundary interface between the light transparent layer 120 and the light absorbing layer 140 is smaller than the critical angle of total reflection, the environment light G is absorbed by the light absorbing layer 140 so as to decrease the visual glare effect caused by the environment light G.
However, if a short focus projector 201 is used to match the projection screen 100, the distance between the short focus projector 201 and the projection screen 100 may be very short. Comparing with the image light beams L1, L2 projected by a common projector 200, a large part of the image light beams L3 projected by the short focus projector 201 is more inclined into the projection screen 100. For the incident angle is smaller than the critical angle of the total reflection, when the image light beams L3 enter into the boundary interface between the light transparent layer 120 and the light absorbing layer 140, the total reflection may not occur and the image light beams L3 may be also absorbed by the light absorbing layer 140, resulting in the invalidation of the projection screen 100.